I have heard the AA back slip on and sounds too loud and raw for my taste, the akra evo is a more sophisticated sound, almost like stock below 3000 rpms but gets very aggressive above, it is a unique sound and the hp you gain with this system is very noticeable, dont think you will get any hp gain with a back slip on exhaust.
Also the akra titanium tips are much better looking than the AA tips but then you get what you pay, the akra evo is much more expensive than the AA